1. Field of the Invention
The instant invention pertains generally to eye movement sensing and more particularly to a method and an apparatus for detecting a rapid eye movement sleep state.
2. Description of the Related Art
It is highly desirable to be able to accurately detect the presence of a rapid eye movement (REM) sleep state. Such detection is valuable for dream analysis, psychotherapy and other medical research.
A variety of methods and apparatus have been designed and applied in an attempt to accurately detect the presence of a rapid eye movement sleep state. An example is disclosed by R. Harper et al., in a publication entitled "A New Technique for Long-Term Recording of Eye Movements in Infants", Vol. 40, No. 1 Electroencephalography and Clinical Neurophysiology, pp. 109-112 (January 1976). In the article, Harper et al. disclose a disk-shaped eye piece which is taped over an eye, such as that of an infant, and which includes a gallium arsenide infrared-emitting diode and a silicon phototransistor mounted side by side to sense movement of one eye during sleep. The diode emits an IR beam which is reflected off of the eye lid or eye surface to the phototransistor. Both the diode and the phototransistor occupy a 0.5 cm.sup.3 package. The device, which senses eye movements of either an open or closed eye, provides an output signal which is recorded on a chart recorder. The chart recorder is visually monitored to determine the presence of REM sleep.
In R. Helford, et al., "REM/NREM Discrimination via Ocular and Limb Movement Monitoring: Correlation with Polygraphic Data and Development of a REM State Algorithm," Psychophysiology, Vol. 23, No. 3, May 1986, pages 334-339, is disclosed a study involving piezoelectric transducers attached to the body in addition to eye movement sensors during sleep assessment. Movements were recorded on a polygraph and an algorithm is described generally for differentiating REM and non-REM periods.
In T. Okuma, et al. "Dream Detector and Automatization of REMP-Awakening Technique for the Study of Dreaming," Psychophysiology, Vol. 7, No. 3, November 1970, pages 508-515, is disclosed a device having electrodes for application to the face of a subject for dream detection. A block diagram in FIG. 1 shows the signal processing steps for the electrode output signals which leads to awakening of the subjects after a number of eye movements. The number of accumulated eye movements over a preset level were the only criteria used in dream detection.
M. Cronin, et al. disclose a system for detecting REM sleep in an article entitled "A Multichannel Hybrid System for Rapid Eye Movement Detection", 23rd A CEMB, Washington, D.C. (Nov. 15-19, 1970). In the system, electrodes are applied to an outer canthus or corner, of an eye. Electrical signals received thereby are filtered to retrieve signals corresponding to eye movements occuring over a period of from about 0.33 to 1.0 second. A threshold criterion is applied to each eye channel to record eye movements greater than a minimum amplitude. A digital output of the system is fed to a computer.
It would be an improvement in the art to provide a simple and inexpensive method and apparatus for automatically detecting and identifying REM sleep with a high degree of accuracy.